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2 Signal transduction
A bacterium has three types of transmembrane receptors, for attractants, repellents and periplasmatic proteins. The signals from these receptors are transmitted across the plasma membrane into the cytosol, where che proteins are activated. The che proteins alter the tumbling frequency, and alter the receptors.
The proteins cheW and cheA bind to the receptor. The activation of the receptor by an external stimulus causes autophosphorylation in cheA, which in turn phosphorylates cheB and cheY. cheY causes tumbling.
cheB, which was activated by cheA, is a methylesterase, removing methyl residues from the cytosolic side of the receptor. It works against cheR, a methyltransferase, which attaches methyl residues to the receptor. The more methyl residues are attached to the receptor, the stronger the signal it gives off. As this signal demethylates the receptor in a feedback loop, the receptor is continuously adjusted to environmental chemical levels, remaining sensitive for small changes even under extreme chemical concentrations.
The behaviour of the bacterium resulting from a basically simple mechanism appears quite complex. The bacterium follows an increasing attractant gradient, but starts changing direction once the concentration of the gradient decreases. This way, it finds the way to the area with the highest concentration of attractant (usually the source) quite well. Even under very high concentrations, it can still distinguish very small differences in concentration. Fleeing from a (poisonous) repellent works with the same efficency. It remains remarkable that this purposeful action is a result of simply choosing between two methods of random movement, namely tumbling and straight swimming.